The present invention is directed to an apparatus and method for dispensing elongated cables into conduits as is required for dispensing electrical cables into underground duct systems.
In the past, electrical power transmission lines were normally supported above the ground by electric poles or towers. The unsightly and potentially dangerous consequences of stringing electrical power lines in this manner occasioned a shift amongst the electric utilities to the dispensing of large quantities of electric power cables in trenches and underground duct systems. Normally, three cables were laid within the trenches and underground duct systems to facilitate the transmission of three phase electrical power.
In U.S. Pat. Nos. 3,788,575, and 3,400,542, apparatus and methods are disclosed to lay cable in trenches wherein a movable assembly supporting a reel having cable wound thereabout had mechanisms to cause rotation of the reel by the application of a torque to the external surface of the reel. These references do not address the substantially more difficult and entirely different problems associated with dispensing cable into underground duct systems from stationary devices.
One method employed by the prior art for dispensing electrical cables into underground duct systems consisted of using three independent reels, each one having a single cable wound thereabout and adapted to be paid out in response to a pulling tension on each cable. A pulling rope was attached to each of the individual cables, thus facilitating the pulling of the cables simultaneously through the underground duct system. Numerous problems arose as a result of this technique. In particular, it was found to be extremely difficult to coordinate dispensing of three cables from three reels simultaneously into an underground duct system. In addition, cumbersome and powerful equipment was necessary to accomplish this technique, even when only one cable was used. In U.S. Pat. Nos. 2,498,834, 2,532,504, 3,363,879, and 4,101,114, apparatus and methods for dispensing cables employing these techniques are described.
Numerous additional problems arose as a result of dispensing cable into underground duct systems as opposed to laying the cable in open trenches. For example, a typical underground duct has an internal diameter of about 4 to 6 inches. The diameter of the cable normally used for the stated purposes is about 13/8ths inches for 350,000 circular mil cable, and about 13/4ths inches for 750,000 to 1,000,000 circular mil cable. The close tolerances between the underground duct width and the accumulated diameter of the multiple cables when placed within the underground duct system caused severe jamming problems in the dispensing process, thus having a potentially deleterious effect on the integrity of the individual electrical cables.
In addition, when using the previous methods and apparatus for laying heavy cable or wire, it was common to employ two or three individuals to aid in paying out the cable during the dispensing operation. These individuals, in addition to controlling the cable payout, also monitored the reel rotation to assure a continuous cable payout and to protect against the potential deleterious effect of the rotational inertia built up by the reels while dispensing cable. It should be apparent that more individuals were required to monitor reel rotation when multiple reels were utilized to dispense cable. It was similarly found that cables were dispensed at varying speeds, thus causing the individuals monitoring reel rotation to vary the reel speeds to occasion more uniform dispersal of the cables into the underground duct systems.
Utility companies attempted to circumvent the problems associated with laying three independent cables in a trench or in an underground duct system by triplexing cable, wherein three lengths of electrical cable were helically twisted about each other with some form of tying wire disposed about the helically twisted cables to assure that they retain to their combined shape. The three lengths of cable comprising the triplex cable were then fitted into a sleeve and each cable was then individually or in combination attached to a pulling rope. The sleeve was found useful in guiding the triplex cable into the underground duct systems.
There were even greater problems with this particular system. For example, triplexing was extremely expensive, costing about 15 to 20 cents per linear foot. Moreover, there were high pulling forces required to draw the triplex cable through the underground duct systems, thus resulting in broken strands and line discontinuities within the individual cables making up the triplex cable. It was also found that there was a substantial wastage of cable when using the triplex configuration because the leftover cable is of little if any use, due to the difficulties in splicing the triplex cable.
In addition, the reels utilized to support the triplex cable were necessarily much larger due to the increased diameter and stiffness of the triplex cable. This situation caused further problems because there were numerous restrictions with regard to the height and width of reels that could be carried upon trucks using public highways. The maximum length of triplex cable that could be carried on a reel to be transported was about 1,100 feet because of the weight and height restrictions imposed by either federal or state highway regulations.
Since this triplex cable could only be carried in lengths up to about 1,100 feet, when laying the triplex cable, it was necessary to build splicing vaults into the underground duct system to join the 1,100 foot cable sections. These splicing vaults are extremely expensive to construct and install, and they are in themselves a source of line discontinuities and insulation problems.
Furthermore, it is well known that triplex cable is particularly sensitive to applied pulling tensions, which potentially can cause a substantial strain in the individual cables and possible breakage of the strands forming the cable or the helically twisted wire. These pulling tensions may be inadvertently occasioned during the process of dispensing the triplex cable from a reel. For example, a 9-foot supply reel fully loaded with triplex cable weighs approximately 17,000 pounds. The application of a sufficient pulling force to overcome the stationary inertia of these reels will often cause damage to the electrical cables being pulled through the underground ducts. Moreover, once the reel is moving the rotational inertia of the reel may also cause an overpayout of cable, thus slowing down the cable dispensing operation. Lastly, uncontrolled reel rotation has the possibility of damaging the cable payout mechanism and potentially creating a hazard to workers operating the equipment dispensing the cable.
In view of the problems associated with pulling triplex cable through an underground duct system or maintaining a uniform flow of cable, if pulling multiple cables individually through an underground duct system, it is clear that there is a need for a system that allows one or more cables to be dispensed into an underground duct system easily, quickly and inexpensively.
There is a further need for a cable dispensing device and method which illustrates the ability to simultaneously monitor the dispensing of multiple cables into an underground duct system.
There is a further need for a cable dispensing device and method which is operative to selectively apply a torque to the rims of the reels from which cable is dispensed into an underground duct system, wherein the reels are subject to a braking force when no pulling tension exists on the cables.
Lastly, there is a need for a device and method which is capable of introducing a plurality of cables into an underground duct system, wherein the device can be moved to the mouth of the duct system despite the terrain and orientation of the duct system into which the cables are to be dispensed.